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From: Yang Shi <yang.shi@linux.alibaba.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-arch <linux-arch@vger.kernel.org>,
	Yu Xu <xuyu@linux.alibaba.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Hillf Danton <hdanton@sina.com>, Hugh Dickins <hughd@google.com>,
	Josef Bacik <josef@toxicpanda.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Linux-MM <linux-mm@kvack.org>,
	mm-commits@vger.kernel.org, Will Deacon <will.deacon@arm.com>,
	Matthew Wilcox <willy@infradead.org>
Subject: Re: [patch 01/15] mm/memory.c: avoid access flag update TLB flush for retried page fault
Date: Mon, 27 Jul 2020 15:43:11 -0700
Message-ID: <89c6671a-39ba-d1cc-9bac-2e6717916220@linux.alibaba.com> (raw)
In-Reply-To: <CAHk-=wha6f0gF1SJg96R77h0oTuc_oO7-37wD=mYGy6TyJOwbQ@mail.gmail.com>


> On Mon, Jul 27, 2020 at 11:04 AM Yang Shi <yang.shi@linux.alibaba.com> wrote:
>> It looks Linus's patch has better data. It seems sane to me since
>> Catalin's patch still needs flush TLB in the shared domain.
> Well, my patch as posted never built at all, I think.
>
> Looking back at that patch, I used FAULT_FLAG_RETRY. But that's not
> the correct name for any of the bits.
>
> So you must have fixed it. Did you make it use "FAULT_FLAG_TRIED"?
> Because that's the right bit - don't flush if this is actually the
> second (or more) attempt.

Actually I didn't have access to that test machine and I didn't try to 
build your patch, Yu Xu helped me test it. I will double check with him 
once he is back online. However that data looks sane since my patch 
(skip pte update) achieved the similar result.

>
> But I'm a bit worried that you would have used one of the other bits
> (FAULT_FLAG_ALLOW_RETRY or FAULT_FLAG_RETRY_NOWAIT), and that would be
> wrong. Those get set on the first attempt to say "you _may_ retry",
> but they get set on the first one.
>
> That just shows how much I tested the patch I sent out. It was
> whitespace-damaged on purpose, but I still want to check.
>
> The "FAULT_FLAG_TRIED" bit I believe is reasonable to test. That one
> literally says "I've gone through this once already, don't bother with
> spurious faults".  But I don't think it triggers much in practice. We
> seldom actually retry faults, it needs a page that we actually start
> IO on (and dropped the mmap lock for) to happen. It wouldn't happen on
> the "turn existing page dirty" case, for example.

With the commit ("mm: drop mmap_sem before calling balance_dirty_pages() 
in write fault") the retried fault may happen much more frequently than 
before since it would drop mmap lock as long as dirty throttling happens.

>
> The "FAULT_FLAG_WRITE" bit is what we test right now. I think it's
> wrong. I think it is a "this happens to work" bit, and cuts down on a
> lot of common cases, by simply skipping something that might be needed
> but basically never is.
>
> So I think a lot of this is dodgy. It doesn't matter on x86, and
> nobody cared. Because x86 will always re-walk the page tables before
> taking an architectural fault (the same way it walks them for
> dirty/accessed bit updates - you could think of x86 as doing all the
> things everybody else does in software, they just do in the hw walker
> micro-fault logic instead).
>
> A local TLB invalidate of a single virtual address should be basically
> free. We're talking single cycles kind of free. The problem here isn't
> the flush_tlb_fix_spurious_fault() itself, the problem here is that
> arm64 (and pretty much everybody else who uses the default fallback)
> does something horribly horribly wrong, and doesn't do the free
> version.

Yes, I do agree global TLB flush seems overkilling for some architectures.

>
>                 Linus



  reply index

Thread overview: 69+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-24  4:14 incoming Andrew Morton
2020-07-24  4:15 ` [patch 01/15] mm/memory.c: avoid access flag update TLB flush for retried page fault Andrew Morton
2020-07-24  4:38   ` Yang Shi
2020-07-24  4:56     ` Andrew Morton
2020-07-24 19:27   ` Linus Torvalds
2020-07-24 20:22     ` Linus Torvalds
2020-07-25  0:36       ` Yang Shi
2020-07-25  1:29         ` Linus Torvalds
2020-07-25 15:58           ` Catalin Marinas
2020-07-28  9:22             ` Will Deacon
2020-07-28  9:39               ` Catalin Marinas
2020-07-28 10:07                 ` Yu Xu
2020-07-28 11:46                   ` Catalin Marinas
2020-07-28 10:21                 ` Will Deacon
2020-07-28 18:28                 ` Linus Torvalds
2020-07-27 17:52           ` Yang Shi
2020-07-27 18:04             ` Linus Torvalds
2020-07-27 18:42               ` Catalin Marinas
2020-07-27 20:56                 ` Linus Torvalds
2020-07-27 22:34               ` Yang Shi
2020-07-27  7:31       ` Yu Xu
2020-07-27 11:05         ` Catalin Marinas
2020-07-27 17:01           ` Linus Torvalds
2020-07-28 11:19             ` Catalin Marinas
2020-07-27 17:12           ` Yu Xu
2020-07-27 18:04             ` Yang Shi
2020-07-27 18:37               ` Linus Torvalds
2020-07-27 22:43                 ` Yang Shi [this message]
2020-07-28  0:38                   ` Linus Torvalds
2020-07-28  0:13                 ` Yu Xu
2020-07-28 10:53                 ` Nicholas Piggin
2020-07-28 19:02                   ` Linus Torvalds
2020-07-28 22:53                     ` Nicholas Piggin
2020-07-29 13:58                       ` Michael Ellerman
2020-07-28  6:41             ` Yu Xu
2020-07-24  4:15 ` [patch 02/15] mm/mmap.c: close race between munmap() and expand_upwards()/downwards() Andrew Morton
2020-07-24  4:15 ` [patch 03/15] vfs/xattr: mm/shmem: kernfs: release simple xattr entry in a right way Andrew Morton
2020-07-24  4:15 ` [patch 04/15] mm: initialize return of vm_insert_pages Andrew Morton
2020-07-24  4:15 ` [patch 05/15] mm/memcontrol: fix OOPS inside mem_cgroup_get_nr_swap_pages() Andrew Morton
2020-07-24  4:15 ` [patch 06/15] mm/memcg: fix refcount error while moving and swapping Andrew Morton
2020-07-24 13:41   ` Alex Shi
2020-07-24  4:15 ` [patch 07/15] mm: memcg/slab: fix memory leak at non-root kmem_cache destroy Andrew Morton
2020-07-24  4:15 ` [patch 08/15] mm/hugetlb: avoid hardcoding while checking if cma is enabled Andrew Morton
2020-07-24  4:15 ` [patch 09/15] khugepaged: fix null-pointer dereference due to race Andrew Morton
2020-07-24  4:15 ` [patch 10/15] mailmap: add entry for Mike Rapoport Andrew Morton
2020-07-24  4:15 ` [patch 11/15] squashfs: fix length field overlap check in metadata reading Andrew Morton
2020-07-24  4:15 ` [patch 12/15] scripts/decode_stacktrace: strip basepath from all paths Andrew Morton
2020-07-24  4:15 ` [patch 13/15] io-mapping: indicate mapping failure Andrew Morton
2020-07-24  4:15 ` [patch 14/15] MAINTAINERS: add KCOV section Andrew Morton
2020-07-24  4:15 ` [patch 15/15] scripts/gdb: fix lx-symbols 'gdb.error' while loading modules Andrew Morton
2020-07-28  1:19 ` mmotm 2020-07-27-18-18 uploaded Andrew Morton
2020-07-28  2:14   ` Stephen Rothwell
2020-07-28  3:22   ` mmotm 2020-07-27-18-18 uploaded (drivers/scsi/ufs/: SCSI_UFS_EXYNOS) Randy Dunlap
2020-07-28  8:23     ` Alim Akhtar
2020-07-28 12:33   ` mmotm 2020-07-27-18-18 uploaded (mm/page_alloc.c) Randy Dunlap
2020-07-28 21:55     ` Andrew Morton
2020-07-28 22:20       ` Stephen Rothwell
2020-07-28 22:31         ` Andrew Morton
2020-07-29 14:18           ` Michael S. Tsirkin
2020-07-29 14:38             ` David Hildenbrand
2020-07-29 16:14               ` David Hildenbrand
2020-07-29 17:29                 ` Randy Dunlap
2020-07-28 22:39       ` Randy Dunlap
2020-07-29  1:43         ` Nathan Chancellor
2020-07-29  1:44         ` Andrew Morton
2020-07-29  2:04           ` Randy Dunlap
2020-07-29 14:09           ` make oldconfig (Re: mmotm 2020-07-27-18-18 uploaded (mm/page_alloc.c)) Alexey Dobriyan
2020-07-31 23:46 ` mmotm 2020-07-31-16-45 uploaded Andrew Morton
2020-08-01  5:24   ` mmotm 2020-07-31-16-45 uploaded (drivers/staging/vc04_services/) Randy Dunlap

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